1. Field of the Invention
The present invention relates to a coil bobbin arranged in a printer head of a dot printer unit, and more specifically to a coil bobbin having a thinner main body to allow longer copper lead wire to be wound thereon. The invention also pertains to a method of molding such a coil bobbin.
2. Description of the Related Art
A typical example of conventional coil bobbin which is disposed in a printer head of a dot printer unit to hit dot pins against a typing sheet is shown in the perspective view of FIG. 8A and the longitudinal sectional view of FIG. 8B. The coil bobbin includes: a tubular main body 101 consisting of a tubular wall having an upper end and a lower end; an upper flange 102 and a lower flange 103 respectively formed on the upper end and the lower end of the main body 101; and a pair of feet 104 and 105 protruded downward from a lower face of the lower flange 103. There are gate marks 106 on the respective outer faces of the feet 104 and 105. Copper lead wire is wound on the circumference of the tubular main body 101 between the upper and lower flanges 102 and 103 to form an electromagnetic coil. The electromagnetic coil is then installed in the printer head via the pair of feet 104 and 105.
Such coil bobbins are generally manufactured by injection molding a thermoplastic resin. FIGS. 9A through 9D are short shot patterns showing a process of injection molding a thermoplastic resin into a coil bobbin with a mold (not shown) having a cavity of a certain shape corresponding to the coil bobbin.
The cavity of the mold is filled with the thermoplastic resin injected through a pair of tunnel gates formed in the mold. The tunnel gates are in contact with a cavity section corresponding to the pair of feet 104 and 105 at the position of the gate marks 106. The thermoplastic resin is injected into the cavity of the mold to form the respective parts of the coil bobbin in the following order: the pair of feet 104 and 105, the lower flange 103, the main body 101, and the upper flange 102, as clearly shown in FIGS. 9A through 9D.
With the recent demand for a smaller printer head, the coil bobbin is required to have a thinner tubular wall, which allows a greater winding number of lead wire with respect to a unit outer area of the tubular wall and thereby improves the response of an electromagnetic coil produced as a final product. When the tubular wall of the coil bobbin has a thickness of 0.1 through 0.2 millimeter, however, undesirable weld marks or air pockets are often observed on the thin tubular wall. Namely, the fraction defective increases and the production yield is worsened.
In the process of injection molding the tubular main body 101 after the pair of feet 104 and 105 and the lower flange 103 as shown in FIG. 9A, the flow of the thermoplastic resin reaches a cavity section corresponding to the upper flange 102 before completing formation of the tubular main body 101. The tubular wall of the main body 101 surrounds a substantially trapezoidal opening as seen in FIG. 8A. The thermoplastic resin successively fills a cavity section of the mold to complete the corners and smaller faces of the substantially trapezoidal tubular wall. The thermoplastic resin, however, starts forming the upper flange 102 before completing the larger faces of the trapezoidal tubular wall as seen in FIG. 9B. This is attributable to the greater thickness of the upper flange 102 than the tubular main body 101.
After completing formation of the upper flange 102, the thermoplastic resin fills a gap 107 formed between the tubular wall 101 and the upper flange 102 shown in FIG. 9C. This may cause a weld mark 108 or air pocket 109 on the thin tubular wall 101 as shown in FIG. 9D.